You’ve probably realized that English is a strange language. Words and phrases can mean one thing in a certain setting, then mean the complete opposite in a different setting. “FINGERS CROSSED” is one of those expressions, as it CAN HAVE A POSITIVE OR NEGATIVE MEANING, depending upon how it’s being used.
If, for example, you have an important test coming up and someone says, “Fingers crossed!” then that person is wishing you good luck on your test, which is very positive thing. On the other hand, if someone promised to do something and then broke that promise, they could say that they had their fingers crossed — meaning they never intended to keep the promise in the first place. (Typically this is something children say if they’ve been caught in a lie.)
If you hear someone use this expression, consider the context. Do they seem to wishing for something positive, or did they fail to do something they promised to do? Hopefully, you’ll only hear people use it in a positive way. Fingers crossed!